1. Field of the Invention
The present invention relates to frequency control procedures within an at least dual-mode mobile radio communications device and, in particular, to such a mobile radio communications device including such frequency control means, and a related method of frequency control.
2. Description of Related Art
As is commonly known, the period between which a mobile radio communications device such as a cell phone handset is turned on, and the time at which the device actually acquires a network for communication purposes, is considered dead time from the user's perspective. This can cause irritation and frustration.
While from a user's perspective this period is considered wasted, the handset is nevertheless actively conducting a search through the variously available frequencies in an attempt to identify a relevant network. This search procedure is one that requires significant power expenditure within the handset. Thus with the advent of dual-mode, and the introduction of multi-mode handsets, there is a correspondingly increasing set of frequencies that have to be searched. The procedure for achieving connection to the most attractive cell of the most attractive network then takes proportionately longer, and thus the related energy consumption is proportionally higher.
While current 3GPP specifications require that a dual mode handset searches one complete Radio Access Technology (RAT) at a time, the relative priority given to the different RATs is generally set within the handset. Thus the usual scenario is that, subsequent to the dual-mode handset being activated by a user, the handset will initially search one specified network and will only commence the search of the other of the network if no suitable cells are located during the search of the first network.
Such initial searches generally seek to measure signal strength, or a derivative thereof, and the cells are effectively ranked in accordance with the strength of their signals as detected by the handset. It is generally required that five measurements are taken for each signal frequency spread over a period of at least three seconds so as to arrive at an average reading.
Considering the EGSM 900 band, which contains 172 frequencies, and GSM 1800 band, which contains 374 frequencies, the following is an illustrative example.
The time taken to tune to a frequency and perform relevant signal strength measurements is in the order of 350 us such that the entire frequency set for the two aforementioned GSM frequency bands can be measured in 0.19 seconds. In order to perform the required five measurements to arrive at an average value, a time interval in the order of 0.95 seconds is therefore required. Given the above-mentioned minimum three second averaging period, this leaves just over two seconds of that period which can be employed for other purposes.
Turning to FIG. 1, there is provided a flow diagram illustrating currently known search procedures which, for simplicity, illustrate the operation according to the current art in accordance with two RATs.
The procedure starts at step 1 with activation of the search and, subsequent to that, Automatic Frequency Control (AFC) is initiated with an initial setting of 0 ppm and a selection of the first RAT to be searched is to be made at step 2.
Depending upon which RAT is selected for the initial searching activity, the procedure continues via steps 3A-7A, or 3B-7B each respective step is however, the same in each of the two series.
That is, subsequent to selection at step 2 of the RAT that is to be searched first, the process continues at step 3A, B with application of the appropriate AFC setting. The search then starts, or continues (see later), at step 4A, B and proceeds to a determination at step 5A, B as to whether the AFC has been updated.
If at step 5A, B the AFC has been updated, then the process returns to step 3A, B so as to apply the updated AFC setting to the relevant RAT prior to continuation of the RAT search.
If, however, at step 5A, B it is determined that there has been no AFC update, the process continues to a determination at step 6A, B as to whether the search phase has been completed. If not, the process returns to step 4A, B for continuation of the RAT search. However if, at step 6A, B it is determined that the search phase has completed, a determination is next made at step 7A, B as to whether all searches for the relevant handset have been completed.
If all such searches have been completed, the process continues to step 8 where it is first determined whether or not a suitable cell has been identified and then, as appropriate, onto steps 9A, B to execute a “camp-on” step 9A, or an “end procedure” step 9B as appropriate.
Returning to step 7A, B, if it is here determined that not all possible searches have been exhausted, i.e. that searches need to be conducted in relation to an alternative RAT, then the procedure returns to step 3A, B as indicated by arrows so that a search phase for the other of the two RATs can be initiated. Once the search phase for both of the RATs is identified at step 7A, B has having been completed, the procedure will then continue to step 8 as noted above.
As an alternative to searching the available RATs in the above-mentioned sequential manner, efficiencies in operation can be realised by effectively interleaving the measurements between the different RATs so as to efficiently employ all the time available to the handset and thereby fill any gaps that might otherwise occur. GB-A-2 395 622 provides a description of such an arrangement.
When performing an initial network search, whether by way of a sequential or interleaved procedure, the control of the frequency of the internal clock of the handset is an important factor in performing the search.
Various AFC techniques have been employed in relation to dual-mode devices such as those discussed in the following.
With a handset logically connected to one of two possible RATs, the internal clock of the handset is locked to a clock signal as originating from the logically connected RAT. This is generally achieved by monitoring the difference between the internal clock and the over-the-air signals received from the RAT. When differences between the clock signals are identified, the local internal clock of the handset is adjusted so as to minimise such differences. Such AFC is commonly employed within an at least dual-mode handset and, at any particular time, the frequency of the internal clock is locked to the over-the-air signals from the RAT to which the handset is logically connected. This network is generally identified as the “master RAT” and if handover is to occur from this network to a different RAT, the current frequency setting of the internal clock is transferred, as a seed value, from the frequency controller of the master RAT to the frequency controller of the RAT, i.e. the slave RAT, to which handover is to be made. The RAT frequency controller previously associated with the slave RAT then takes control of the frequency of the internal clock using the clock value as transferred to it as a seed value.
For example, such an arrangement is known from GB-A-2 387 507. This technique takes advantage of the realisation that the transferred setting is known to be within the accuracy limits permissible in the slave RAT.
Of course, when the handset is first activated, and an initial network search is to be performed, there is no accurate value available to be used as a seed value since no network connection has yet been established. Without the benefit of a current network connection, each RAT generally has its own requirements and arrangements as to how the AFC should operate.
As an example, GSM requires the value to be fixed throughout the search procedure and generally relies upon an assumption that any clock frequency inaccuracy will be within the range of reception of the DSP equaliser software within the handset.
As a comparison, a WCDMA equaliser is known to be processing intensive even when running with an accurate clock signal and this translates into a requirement that the error should be relatively small, and generally smaller than those errors that are permissible for GSM systems. In view of this, and in order to search effectively for an initial network connection, it is necessary to raster the AFC setting during the search so as to ensure that all possible errors have been allowed for. For example, with a device handset with an internal clock accurate to ±6 ppm in the absence of AFC, and an equaliser capable of dealing with ±2 ppm error, the AFT correction would then be set to a nominal value, i.e. 0 ppm. For the first of the raster scans, this would allow the equaliser to analyse the range +2 ppm −2 ppm. For the second raster scan, the AFC would then be set to +4 ppm thus covering the range +2 ppm to +6 ppm. The final scan AFC would be −4 ppm and the equaliser would then be able to cover the range −2 ppm to −6 ppm.
As will be appreciated, these initial search procedures for the different RATs are quite different and exhibit quite different characteristics.
These approaches clearly lack compatibility particularly when considering interleaved RAT measurement scanning, and in a situation in which a currently active RAT has control of the AFC.
In particular, since there is currently no exchange of data between different RATs concerning their respective current AFC settings, when control moves from one RAT to the other, the search procedure adopted by each respective RAT maintains its own setting, and applies that setting when it is in control of the radio operation.
Since, as noted above, a 3G RAT will apply a raster step during its search procedure, whereas a 2G RAT will not apply any such step, the 3G section can be set to a +4 ppm (or −4 ppm) nominal value, while the 2G section will of course require use of a 0 ppm nominal value.
In view of this, and each time the RAT changes, the AFC would then have to undergo a step-change of 4 ppm. The time required to allow the internal handset clock to settle after such a relatively large change would prove particularly disadvantageous. In extremis, the clock setting time may be longer than the interval available to perform measurements, and attempting to interleave measurements becomes impossible.